It sure didn’t take long for some significant injuries at NFL training camps — Eagles receiver Jeremy Maclin, Ravens tight end Dennis Pitta, Broncos center Dan Koppen, to name a few.

Immediately, some theories developed: too much offseason work. Not enough. New labor-contract rules limiting padded practices to one per day, while generally seen as helpful, are hardly a cure-all.

Redskins linebacker London Fletcher said he thinks some guys get hurt in camp because players are trying so hard to impress coaches and earn a roster spot or a starting job.

“You know now coaches are really evaluating you,” said Fletcher, whose teammate, second-year linebacker Keenan Robinson, tore his left pectoral muscle on the first day of training camp. “You’ve got guys with a competitive spirit and they’re looking at it like, ‘My job’s on the line. I need to make a play.’ “

Whatever the cause, severe injuries are increasing in the NFL lately. The number of injuries that forced a player to miss at least eight days jumped every year from 2009 to 2012, according to an analysis of NFL injury data released Wednesday.

The study by Edgeworth Economics, based on information collected by the league, also shows players with concussions missed an average of 16 days last season, up from only four days in 2005, while the length of time out for other types of injuries has been steadier.

“Severe injuries are increasing in frequency,” Jesse David, the economist overseeing the study, said. “I know that’s a very important issue for both the players association and the league — trying to tweak the rules and the equipment to deal with that. But despite everything they’ve been doing, it’s still going on.”

The study says there were 1,095 instances of injuries sidelining a player for eight or more days in 2009 — including practices and games in the preseason, regular season and postseason — and that climbed to 1,272 in 2010, 1,380 in 2011 and 1,496 in 2012. That’s an increase of 37 percent.

Concussions have become a more noticeable part of football in recent years, with more discussion of the links between head injuries and brain disease, hundreds of lawsuits brought by thousands of former players and rules changes made by the NFL to try to better protect players.

During the nine years examined in David’s study, the average number of days missed because of head injuries by players in the league went from 4.8 in 2004, four in 2005 and 4.1 in 2006 to 10.9 in 2010, 12 in 2011 and 16 last season.

“I think there’s more awareness in a lot of different areas when it comes to injuries over the last few years, and rightfully so,” Redskins coach Mike Shanahan said.

NFL spokesman Brian McCarthy, who said the league will look at the study’s findings, attributed the longer absences for players with concussions to more caution in the treatment of those types of injuries.

KANSAS CITY, Mo. — Tyreek Hill had touchdowns receiving and on a punt return, Kansas City’s defense made life miserable for Oakland quarterback Derek Carr, and the Chiefs beat the Raiders 21-13 on a frigid Thursday night to take control of the AFC West. Charcandrick West also had a touchdown run for the Chiefs (10-3). They moved into a first-place...